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 // Created on: 1993-02-17
 // Created by: Remi LEQUETTE
 // Copyright (c) 1993-1999 Matra Datavision
 // Copyright (c) 1999-2014 OPEN CASCADE SAS
 //
 // This file is part of Open CASCADE Technology software library.
 //
 // This library is free software; you can redistribute it and/or modify it under
 // the terms of the GNU Lesser General Public License version 2.1 as published
 // by the Free Software Foundation, with special exception defined in the file
 // OCCT_LGPL_EXCEPTION.txt. Consult the file LICENSE_LGPL_21.txt included in OCCT
 // distribution for complete text of the license and disclaimer of any warranty.
 //
 // Alternatively, this file may be used under the terms of Open CASCADE
 // commercial license or contractual agreement.
 
 #ifndef _Precision_HeaderFile
 #define _Precision_HeaderFile
 
 #include <Standard.hxx>
 #include <Standard_DefineAlloc.hxx>
 #include <Standard_Real.hxx>
 
 //! The Precision package offers a set of functions defining precision criteria
 //! for use in conventional situations when comparing two numbers.
 //! Generalities
 //! It is not advisable to use floating number equality. Instead, the difference
 //! between numbers must be compared with a given precision, i.e. :
 //! Standard_Real x1, x2 ;
 //! x1 = ...
 //! x2 = ...
 //! If ( x1 == x2 ) ...
 //! should not be used and must be written as indicated below:
 //! Standard_Real x1, x2 ;
 //! Standard_Real Precision = ...
 //! x1 = ...
 //! x2 = ...
 //! If ( Abs ( x1 - x2 ) < Precision ) ...
 //! Likewise, when ordering floating numbers, you must take the following into account :
 //! Standard_Real x1, x2 ;
 //! Standard_Real Precision = ...
 //! x1 = ...       ! a large number
 //! x2 = ...       ! another large number
 //! If ( x1 < x2 - Precision ) ...
 //! is incorrect when x1 and x2 are large numbers ; it is better to write :
 //! Standard_Real x1, x2 ;
 //! Standard_Real Precision = ...
 //! x1 = ...       ! a large number
 //! x2 = ...       ! another large number
 //! If ( x2 - x1 > Precision ) ...
 //! Precision in Cas.Cade
 //! Generally speaking, the precision criterion is not implicit in Cas.Cade. Low-level geometric algorithms accept
 //! precision criteria as arguments. As a rule, they should not refer directly to the precision criteria provided by the
 //! Precision package.
 //! On the other hand, high-level modeling algorithms have to provide the low-level geometric algorithms that they
 //! call, with a precision criteria. One way of doing this is to use the above precision criteria.
 //! Alternatively, the high-level algorithms can have their own system for precision management. For example, the
 //! Topology Data Structure stores precision criteria for each elementary shape (as a vertex, an edge or a face). When
 //! a new topological object is constructed, the precision criteria are taken from those provided by the Precision
 //! package, and stored in the related data structure. Later, a topological algorithm which analyses these objects will
 //! work with the values stored in the data structure. Also, if this algorithm is to build a new topological object, from
 //! these precision criteria, it will compute a new precision criterion for the new topological object, and write it into the
 //! data structure of the new topological object.
 //! The different precision criteria offered by the Precision package, cover the most common requirements of
 //! geometric algorithms, such as intersections, approximations, and so on.
 //! The choice of precision depends on the algorithm and on the geometric space. The geometric space may be :
 //! -   a "real" 2D or 3D space, where the lengths are measured in meters, millimeters, microns, inches, etc ..., or
 //! -   a "parametric" space, 1D on a curve or 2D on a surface, where lengths have no dimension.
 //! The choice of precision criteria for real space depends on the choice of the product, as it is based on the accuracy
 //! of the machine and the unit of measurement.
 //! The choice of precision criteria for parametric space depends on both the accuracy of the machine and the
 //! dimensions of the curve or the surface, since the parametric precision criterion and the real precision criterion are
 //! linked : if the curve is defined by the equation P(t), the inequation :
 //! Abs ( t2 - t1 ) < ParametricPrecision
 //! means that the parameters t1 and t2 are considered to be equal, and the inequation :
 //! Distance ( P(t2) , P(t1) ) < RealPrecision
 //! means that the points P(t1) and P(t2) are considered to be coincident. It seems to be the same idea, and it
 //! would be wonderful if these two inequations were equivalent. Note that this is rarely the case !
 //! What is provided in this package?
 //! The Precision package provides :
 //! -   a set of real space precision criteria for the algorithms, in view of checking distances and angles,
 //! -   a set of parametric space precision criteria for the algorithms, in view of checking both :
 //! -   the equality of parameters in a parametric space,
 //! -   or the coincidence of points in the real space, by using parameter values,
 //! -   the notion of infinite value, composed of a value assumed to be infinite, and checking tests designed to verify
 //! if any value could be considered as infinite.
 //! All the provided functions are very simple. The returned values result from the adaptation of the applications
 //! developed by the Open CASCADE company to Open CASCADE algorithms. The main interest of these functions
 //! lies in that it incites engineers developing applications to ask questions on precision factors. Which one is to be
 //! used in such or such case ? Tolerance criteria are context dependent. They must first choose :
 //! -   either to work in real space,
 //! -   or to work in parametric space,
 //! -   or to work in a combined real and parametric space.
 //! They must next decide which precision factor will give the best answer to the current problem. Within an application
 //! environment, it is crucial to master precision even though this process may take a great deal of time.
 class Precision 
 {
 public:
 
   DEFINE_STANDARD_ALLOC
 
   //! Returns the recommended precision value
   //! when checking the equality of two angles (given in radians).
   //! Standard_Real Angle1 = ... , Angle2 = ... ;
   //! If ( Abs( Angle2 - Angle1 ) < Precision::Angular() ) ...
   //! The tolerance of angular equality may be used to check the parallelism of two vectors :
   //! gp_Vec V1, V2 ;
   //! V1 = ...
   //! V2 = ...
   //! If ( V1.IsParallel (V2, Precision::Angular() ) ) ...
   //! The tolerance of angular equality is equal to 1.e-12.
   //! Note : The tolerance of angular equality can be used when working with scalar products or
   //! cross products since sines and angles are equivalent for small angles. Therefore, in order to
   //! check whether two unit vectors are perpendicular :
   //! gp_Dir D1, D2 ;
   //! D1 = ...
   //! D2 = ...
   //! you can use :
   //! If ( Abs( D1.D2 ) < Precision::Angular() ) ...
   //! (although the function IsNormal does exist).
   static Standard_Real Angular() { return 1.e-12; }
 
   //! Returns the recommended precision value when
   //! checking coincidence of two points in real space.
   //! The tolerance of confusion is used for testing a 3D
   //! distance :
   //! -   Two points are considered to be coincident if their
   //! distance is smaller than the tolerance of confusion.
   //! gp_Pnt P1, P2 ;
   //! P1 = ...
   //! P2 = ...
   //! if ( P1.IsEqual ( P2 , Precision::Confusion() ) )
   //! then ...
   //! -   A vector is considered to be null if it has a null length :
   //! gp_Vec V ;
   //! V = ...
   //! if ( V.Magnitude() < Precision::Confusion() ) then ...
   //! The tolerance of confusion is equal to 1.e-7.
   //! The value of the tolerance of confusion is also used to
   //! define :
   //! -   the tolerance of intersection, and
   //! -   the tolerance of approximation.
   //! Note : As a rule, coordinate values in Cas.Cade are not
   //! dimensioned, so 1. represents one user unit, whatever
   //! value the unit may have : the millimeter, the meter, the
   //! inch, or any other unit. Let's say that Cas.Cade
   //! algorithms are written to be tuned essentially with
   //! mechanical design applications, on the basis of the
   //! millimeter. However, these algorithms may be used with
   //! any other unit but the tolerance criterion does no longer
   //! have the same signification.
   //! So pay particular attention to the type of your application,
   //! in relation with the impact of your unit on the precision criterion.
   //! -   For example in mechanical design, if the unit is the
   //! millimeter, the tolerance of confusion corresponds to a
   //! distance of 1 / 10000 micron, which is rather difficult to measure.
   //! -   However in other types of applications, such as
   //! cartography, where the kilometer is frequently used,
   //! the tolerance of confusion corresponds to a greater
   //! distance (1 / 10 millimeter). This distance
   //! becomes easily measurable, but only within a restricted
   //! space which contains some small objects of the complete scene.
   static Standard_Real Confusion() { return 1.e-7; }
 
   //! Returns square of Confusion.
   //! Created for speed and convenience.
   static Standard_Real SquareConfusion() { return Confusion() * Confusion(); }
 
   //! Returns the precision value in real space, frequently
   //! used by intersection algorithms to decide that a solution is reached.
   //! This function provides an acceptable level of precision
   //! for an intersection process to define the adjustment limits.
   //! The tolerance of intersection is designed to ensure
   //! that a point computed by an iterative algorithm as the
   //! intersection between two curves is indeed on the
   //! intersection. It is obvious that two tangent curves are
   //! close to each other, on a large distance. An iterative
   //! algorithm of intersection may find points on these
   //! curves within the scope of the confusion tolerance, but
   //! still far from the true intersection point. In order to force
   //! the intersection algorithm to continue the iteration
   //! process until a correct point is found on the tangent
   //! objects, the tolerance of intersection must be smaller
   //! than the tolerance of confusion.
   //! On the other hand, the tolerance of intersection must
   //! be large enough to minimize the time required by the
   //! process to converge to a solution.
   //! The tolerance of intersection is equal to :
   //! Precision::Confusion() / 100.
   //! (that is, 1.e-9).
   static Standard_Real Intersection() { return Confusion() * 0.01; }
 
   //! Returns the precision value in real space, frequently used
   //! by approximation algorithms.
   //! This function provides an acceptable level of precision for
   //! an approximation process to define adjustment limits.
   //! The tolerance of approximation is designed to ensure
   //! an acceptable computation time when performing an
   //! approximation process. That is why the tolerance of
   //! approximation is greater than the tolerance of confusion.
   //! The tolerance of approximation is equal to :
   //! Precision::Confusion() * 10.
   //! (that is, 1.e-6).
   //! You may use a smaller tolerance in an approximation
   //! algorithm, but this option might be costly.
   static Standard_Real Approximation() { return Confusion() * 10.0; }
 
   //! Convert a real  space precision  to  a  parametric
   //! space precision.   <T>  is the mean  value  of the
   //! length of the tangent of the curve or the surface.
   //!
   //! Value is P / T
   static Standard_Real Parametric (const Standard_Real P, const Standard_Real T) { return P / T; }
 
   //! Returns a precision value in parametric space, which may be used :
   //! -   to test the coincidence of two points in the real space,
   //! by using parameter values, or
   //! -   to test the equality of two parameter values in a parametric space.
   //! The parametric tolerance of confusion is designed to
   //! give a mean value in relation with the dimension of
   //! the curve or the surface. It considers that a variation of
   //! parameter equal to 1. along a curve (or an
   //! isoparametric curve of a surface) generates a segment
   //! whose length is equal to 100. (default value), or T.
   //! The parametric tolerance of confusion is equal to :
   //! -   Precision::Confusion() / 100., or Precision::Confusion() / T.
   //! The value of the parametric tolerance of confusion is also used to define :
   //! -   the parametric tolerance of intersection, and
   //! -   the parametric tolerance of approximation.
   //! Warning
   //! It is rather difficult to define a unique precision value in parametric space.
   //! -   First consider a curve (c) ; if M is the point of
   //! parameter u and M' the point of parameter u+du on
   //! the curve, call 'parametric tangent' at point M, for the
   //! variation du of the parameter, the quantity :
   //! T(u,du)=MM'/du (where MM' represents the
   //! distance between the two points M and M', in the real space).
   //! -   Consider the other curve resulting from a scaling
   //! transformation of (c) with a scale factor equal to
   //! 10. The 'parametric tangent' at the point of
   //! parameter u of this curve is ten times greater than the
   //! previous one. This shows that for two different curves,
   //! the distance between two points on the curve, resulting
   //! from the same variation of parameter du, may vary   considerably.
   //! -   Moreover, the variation of the parameter along the
   //! curve is generally not proportional to the curvilinear
   //! abscissa along the curve. So the distance between two
   //! points resulting from the same variation of parameter
   //! du, at two different points of a curve, may completely differ.
   //! -   Moreover, the parameterization of a surface may
   //! generate two quite different 'parametric tangent' values
   //! in the u or in the v parametric direction.
   //! -   Last, close to the poles of a sphere (the points which
   //! correspond to the values -Pi/2. and Pi/2. of the
   //! v parameter) the u parameter may change from 0 to
   //! 2.Pi without impacting on the resulting point.
   //! Therefore, take great care when adjusting a parametric
   //! tolerance to your own algorithm.
   static Standard_Real PConfusion (const Standard_Real T) { return Parametric (Confusion(), T); }
 
   //! Returns square of PConfusion.
   //! Created for speed and convenience.
   static Standard_Real SquarePConfusion() { return PConfusion() * PConfusion(); }
 
   //! Returns a precision value in parametric space, which
   //! may be used by intersection algorithms, to decide that
   //! a solution is reached. The purpose of this function is to
   //! provide an acceptable level of precision in parametric
   //! space, for an intersection process to define the adjustment limits.
   //! The parametric tolerance of intersection is
   //! designed to give a mean value in relation with the
   //! dimension of the curve or the surface. It considers
   //! that a variation of parameter equal to 1. along a curve
   //! (or an isoparametric curve of a surface) generates a
   //! segment whose length is equal to 100. (default value), or T.
   //! The parametric tolerance of intersection is equal to :
   //! -   Precision::Intersection() / 100., or Precision::Intersection() / T.
   static Standard_Real PIntersection (const Standard_Real T) { return Parametric(Intersection(),T); }
 
   //! Returns a precision value in parametric space, which may
   //! be used by approximation algorithms. The purpose of this
   //! function is to provide an acceptable level of precision in
   //! parametric space, for an approximation process to define
   //! the adjustment limits.
   //! The parametric tolerance of approximation is
   //! designed to give a mean value in relation with the
   //! dimension of the curve or the surface. It considers
   //! that a variation of parameter equal to 1. along a curve
   //! (or an isoparametric curve of a surface) generates a
   //! segment whose length is equal to 100. (default value), or T.
   //! The parametric tolerance of intersection is equal to :
   //! -   Precision::Approximation() / 100., or Precision::Approximation() / T.
   static Standard_Real PApproximation (const Standard_Real T) { return Parametric(Approximation(),T); }
 
   //! Convert a real  space precision  to  a  parametric
   //! space precision on a default curve.
   //!
   //! Value is Parametric(P,1.e+2)
   static Standard_Real Parametric (const Standard_Real P) { return Parametric (P, 100.0); }
 
   //! Used  to test distances  in parametric  space on a
   //! default curve.
   //!
   //! This is Precision::Parametric(Precision::Confusion())
   static Standard_Real PConfusion() { return Parametric (Confusion()); }
 
   //! Used for Intersections  in parametric  space  on a
   //! default curve.
   //!
   //! This is Precision::Parametric(Precision::Intersection())
   static Standard_Real PIntersection() { return Parametric (Intersection()); }
 
   //! Used for  Approximations  in parametric space on a
   //! default curve.
   //!
   //! This is Precision::Parametric(Precision::Approximation())
   static Standard_Real PApproximation() { return Parametric (Approximation()); }
 
   //! Returns True if R may be considered as an infinite
   //! number. Currently Abs(R) > 1e100
   static Standard_Boolean IsInfinite (const Standard_Real R) { return Abs (R) >= (0.5 * Precision::Infinite()); }
 
   //! Returns True if R may be considered as  a positive
   //! infinite number. Currently R > 1e100
   static Standard_Boolean IsPositiveInfinite (const Standard_Real R) { return R >= (0.5 * Precision::Infinite()); }
 
   //! Returns True if R may  be considered as a negative
   //! infinite number. Currently R < -1e100
   static Standard_Boolean IsNegativeInfinite (const Standard_Real R) { return R <= -(0.5 * Precision::Infinite()); }
 
   //! Returns a  big number that  can  be  considered as
   //! infinite. Use -Infinite() for a negative big number.
   static Standard_Real Infinite() { return 2.e+100; }
 
 };
 
 #endif // _Precision_HeaderFile

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